Now the spotlight swings to John McCain, who emerged from Super Tuesday with a commanding lead for the Republican nomination. Even though Democrats will enter the fall campaign as heavy favorites, McCain is fully capable of mounting a serious challenge. But to do so, he has to answer two large questions.

The first is political. Can he appease his conservative critics while continuing his appeal to moderates and independents? Can he run on promises of bipartisan cooperation without alienating ideologues that view such talk as heresy?

The second is personal. Can he satisfy voters that his age, health and temper do not disqualify him from the Oval Office? Can a 70-something cancer survivor, with a long history of flaying his rivals with bitter invective, survive the intense media scrutiny he is about to endure?

All the excitement and energy this year is on the Democratic side. Before the California primary, 150,000 voters registered as Democrats and only 40,000 joined the Republicans. Without campaigning in a meaningless Florida primary, Hillary Clinton drew 160,000 more votes than McCain.

History, however, waves a caution flag. The only successful Democrats since 1968, Jimmy Carter and Bill Clinton, were moderate Southern governors. This year the party will nominate a liberal senator from either New York or Illinois. Before then, Clinton and Barack Obama face weeks, even months, of debilitating civil (or uncivil) warfare. McCain can start healing wounds while his rivals are still picking scabs.

But to take advantage of this opening, McCain must overcome some huge obstacles. The GOP has been so successful because it has united three diverse strands of conservatism: evangelical Christians in the South, libertarians in the West and fiscal/business types in the Middle West and Northeast. Ronald Reagan and the two Bushes held that coalition together through five winning campaigns, but it is now starting to fray badly.

McCain has a strongly conservative voting record, particularly on Iraq, but he’s poked a finger in the eye of each group. A lot of Westerners don’t like his pro-immigration policy, a problem that showed up in California; the fiscal warriors resent his rejection of the Bush tax cuts. Above all, the evangelicals don’t think he shares their passionate devotion to social issues, and they’re right. That’s why he called leaders of the Religious Right “agents of intolerance” during the 2000 election.

Given all these conflicts, only one in three conservatives voted for McCain on Tuesday; among “very” conservative voters, he won only one in five.

Going forward, McCain faces a classic “peas on the knife” problem. He appeals to moderates and independents by saying, as he did in Connecticut, “I will reach across the aisle to the Democrats and work together for the good of the country.”